Why do we need tiedowns:

  Most telescope pointing errors are repeatable. These include beam deflections caused by the motion of  the azimuth and dome. Using a pointing model (made by tracking sources), we can get the pointing error of the telescope down to about 5 arc seconds rms. This is needed since the FWHM beam at xband is 25 arcseconds,.

    A 5 arcsecond error is a motion of 3 mm at the feed. The repeatability of the errors allows us to position the horns in absolute space to about 3mm.

    The one pointing error that is not repeatable is temperature. When the main cables heat up, they expand, when they cool off ,they  contract. During the night times, the telescope will move vertically 1 inch for every 5 degree temperature change

• Note: We currently measure the air temperature. The thermal motion of the cables depends on the cable temperature. At night, the cable and air temperature are in equilibrium. During the day the cable temperature can be 10 to 20 degrees above the air temperature (at least thats what jim gould from amahan and whitney has mentioned).  So when i quote numbers for  temperature dependence, they will  be valid for nighttime, and for some hours of daytime.   

The tiedown system:

• 3 jacks located under the dish below each platform corner.
  
• Each tiedown jack is connected to the platform corner above it with a 1.5 inch wire rope (about 500 feet long).
  
• The jacks (under computer control)  can apply up the 120000 lbs of tension on each corner of the platform.
• A laser ranging (distomat) system measures the average platform height.
• 6 distomats around the edge of the dish measure the distance to the platform corners every two minutes.
• A tiedown correction is computer from the measured  platform height and then sent to the tiedown computers to move the jacks.

Constraints in using the tiedown system.

• The required platforn height is fixed by the optics.
• The jack throw is limited to about 20 inches.
  
• The tiedown cables stretch as they are pulled: 1.7 inch tiedown motion gives 1 inch of platform motion. So the 22 inches of jack throw is about 11.8 inches of platform motion.
• temperature variation has 5 degF moving the platform 1 inch. The 11.8 inches of platform motion maps to 60 degF of temperature variations
  
• If the temperature goes down, the main cables decrease in length, so the platform rises. We then pull on the tiedowns.
• The maximum tension we can apply from one tiedown is 120kips (1 kip = 1000 lbs).
• The maximum total we can apply from all tiedowns combined is 240 kips.
• If the temperature goes up, the main cables get longer and the platform sinks. We release tension in the tiedowns to move the platform back up.
• If we ever lose tension in a tiedown cable, then we loose the ability to move the platform up (you can't push on a wet noodle).
• We run an unbalanced system (see platform weight distribution .gif)
  
•  The dome weighs 220 kips.
  
• The carriage house (on the opposite side of the azimuth arm) weights 35kips. When using the dome we leave the carriage house parked at 8.8 degrees za.
  
•  There is a fixed counterweight of 45 kips at the end of the carriage house side of the azimuth arm.
• The tension in a tiedown cable can change if the azimuth points at the tiedown (it goes as cos(az - azTiedown)), and then the dome moves in za.
• If the dome moves to a high enough za, we will lose tension in a tiedown cable (even if all of the other tiedown cable have tension).
• This is the major reason why we lose tension in a tiedown cable...
• If the platform is lighter (higher) then this problem occurs less often.

Problems with using the tiedown system:

• The tiedown cables are the main path to ground for the tower/platform. Lightning striking the towers/platform goes through the tiedown cables when going to ground (see a partial list of tiedown failures.)
  
• When lightning passes through the tiedown cables, the electrical equipement (cpu, i/o boards, motor power supply and ampiflier, encoders, load cell amplifiers) can be damaged. Lightning is a frequent occurrence at arecibo may through October.
• The cpu and i/o boards used for the tiedowns are no longer manufactured.
•  We recently ran out of cpu boards after a lightning strike in oct11 destroyed 3 cpus and multiple i/o boards. It took us 3 months before we found a vendor who made a one time run to build cpu  (30) and some i/o boards. We now (2012) have a good supply of cpus and most i/o boards. There is one i/o board that the vendor could not build. We only have 5 spares for this one.
• Even with the spares, the lightning strikes require a lot of man hours to debug and replace the components of the system (it's not always as easy as ...replace the chips with the holes in them).

Replacing the tiedown cables with kevlar cables.

• Kevlar is an insulator. This would stop the lightning from using the tiedown cables as the path to ground.
• It's' been mentioned that a wet kevlar cable would still conduct electricity through the water on the outside.
• The cables themselves will have to be protected from the UV by some material. We currently use a plastic cover on the linefeed guy wires which are kevlar (The plastic had been replace a few times over the years they've been up there).
• A lightning pulse lasts for 10-20 microseconds with 20-30 kilioAmp currents and KiloVolt voltages.
• A thin layer of water on a plastic coat may conduct electricity, but i doubt it could support any current before the water is vaporized. As long at the air around the cable is not ionized, there should not be an appreciable current flow.
  
• There are multiple electrical paths to ground from the platform/towers (ground cable from tower to earth, cement towers with rebar, power cables from platform). Currently the 1.5 inch tiedown cables are the lowest resistance path to ground. By switching to a wet kevlar cable, it doubt  that this would still be the case (ok.. then we have to worry about what the lightning does on the other paths to ground).
• Kevlar cables would weigh 11000 lbs less than the current tiedown cables. If you think of the tiedown cables as part of the platform, then the platform would weigh 11000 lbs less.
  
• For the same cold temperature, you would now have to pull with 11000 more pounds (11000/3 per tiedown) to put the platform into focus.
• Platform motion with weight:
• When there is no tension in the tiedowns, the platform will move 1 inch when 8000 lbs are added to it.
•  11000 lbs would move the platform 1.4 Inches (about the same as 7 deg F).
  
• When the tiedowns still have tension, it takes 24000 lbs to  move the platform 1 inch.
• In this case, the platform moves down and loses some of the tension  that was in the tiedown cables. You need the 16000 extra lbs to make up for this loss.
• In this case 11000 lbs would move the platform up about .5 inches. The same as 2.5 degF drop in temperature.
• When the dome goes up to a high za and points near a tiedown, we would now have about 4000 more lbs of tension before we would lose tension in a tiedown.
• For za motion of dome (with td tension), the platform corner moves .46 inches for 1 degree of  dome za motion.
  
• for no td tension,11 kips gives us 1.4  platform inchs, or about 3 extra za degrees of motion. 2 za degrees.
• For td tension 11 kips gives about 1 extra degree of za range for the dome.
• How often do we see lose tension in a tiedown.
• The plots shows the measured total tiedown tension versus the air temperature for a few days in march 2012 (.png)
• These points are selected so that the platform is still in focus.
  
• The colored horizontal  lines show dome za's  where a tiedown cable would lose tension  if the azimuth pointed at a tiedown. The az dependence goes as cos(azTd -az) so you still have problems when you don't point at a tiedown.
• daytime observing may thru sept usually has temperatures > 80 degrees (see platform yearly temperature plots)
  

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